Invasive Asian Tiger Species, Now in the U.S., Has Scientists Worried About Ecosystem

BAYOU LA BATRE, Ala.—Dennis Morrison took a boat out with a friend recently to cast nets for white shrimp in the marshes around this seafood port. On one of his last throws, an odd-looking black shrimp came up in the haul. It didn't act like the others; it jumped and flopped in the boat.

"I never seen nothing like this since I've been around shrimping," said Mr. Morrison, 49 years old, who worked in the shrimp industry...